Bibliothek

Notizen: We have studied the effect of misfit strain on the microstructure and properties of ferroelectric lead zirconate titanate thin films. We have changed the misfit strain by varying the film thickness and studied the thickness effect on the domain formation of epitaxial PbZr0.2Ti0.8O3 (PZT) films grown by pulsed laser deposition on (001) LaAlO3 substrates with La0.5Sr0.5CoO3 (LSCO) electrodes. The nominal thickness of the PZT films was varied from 60 to 400 nm with the LSCO electrode thickness kept constant at 50 nm. X-ray diffraction experiments show that the films relax via the formation of a domains, the fraction of which increase with the ferroelectric film thickness. The c-axis lattice constant of PZT films calculated from the 002 reflection decreases with increasing film thickness and approaches the bulk value of ∼0.413 nm in the films thicker than 300 nm. Cross-sectional transmission electron microscopy images reveal that the a-domain fraction and period increase with increasing film thickness. The relaxation of misfit strain in the film is accompanied by systematic changes in the polarization properties, as well as the switching fields, quantified by the coercive field and the activation field. © 1999 American Institute of Physics.

Notizen: The operational limits observed in spherical tokamaks, notably the small tight aspect ratio tokamak (START) device [A. Sykes et al., Nucl. Fusion 32, 694 (1992)], are consistent with those found in conventional aspect ratio tokamaks. In particular the highest β achieved (∼40%) is consistent with an ideal magneto-hydro-dynamic (MHD) Troyon type limit, the upper limit on density is well described by the Greenwald density (πa2n¯e/Ip∼1) and the normalized current (Ip/aBt) is limited such that q95(approximately-greater-than)2. Stability calculations indicate scope for increasing both normalized β and normalized current beyond the values so far achieved, although wall stabilization is generally needed for low-n modes. In double null configurations current terminating disruptions occur at each of the operational boundaries, though the current quench tends to be slow at the density limit and disruptions at high β may be due to the low q. In early limiter START discharges, before the divertor coils were installed, disruptions rarely occurred. Instead internal reconnection events which have all the characteristics of a disruption except the current quench occurred. These various disruptive behaviors are explained in terms of a model in which helicity is conserved during the disruption. Due to the low toroidal field beam ions in START, and α particles in a ST power plant, are super-Alfvénic. This gives the possibility for toroidal Alfvén eigenmodes (TAEs) to occur and such modes are frequently observed in START neutral beam injection (NBI) discharges, but seem to be benign. The features of these observed TAEs are shown to be in agreement with MHD calculations.

Notizen: Low-aspect-ratio tokamaks offer both the economic advantage of smaller size and a number of physics advantages which are not available at conventional aspect ratio. The Small Tight Aspect Ratio Tokamak (START) [Fusion Technology 1990, edited by B. E. Keen, M. Huguet, and R. Hemsworth (North-Holland, Amsterdam, 1991), Vol. 1, p. 353] was conceived as a first substantial test of tokamak plasma behavior at low aspect ratio. It has achieved plasma currents up to 200 kA, peak densities of ∼2×1020 m−3 and central electron temperatures of ∼500 eV at an aspect ratio of 1.3–1.5. Central beta values of ∼13% have been measured and the volume-averaged beta 〈β〉 can approach the Troyon limit. Plasmas are naturally elongated (κ(approximately-less-than)2.0) and are vertically stable without feedback control. Major disruptions have not been observed at low aspect ratios (A≤2.0).

Notizen: In this letter, we report on the influence of lead content on thin-film ferroelectric properties of lead niobium zirconate titanate. These films were prepared by the sol-gel technique and deposited on (La,Sr)CoO3 electrodes. It was determined that 7% excess lead in the sol was required to obtain nominally stoichiometric films. Lead deficiency in the film results in lead vacancies and excess lead is accommodated by forming octahedral site vacancies. Further amounts of lead in the sol leads to second phase PbO, which then coexists with the perovskite phase. The charged vacancies are compensated by mobile holes, which can interact with domains during switching. Under applied field and short pulse widths, the films with larger number of holes exhibited poor switching. Significant polarization relaxation was measured for films with excess lead, which is attributed to interaction of ionic defects with domains. Our results indicate that lead excess leads to poor reliability properties,whereas lead deficiency suppresses the polarization of the capacitors. © 1999 American Institute of Physics.

Notizen: We report on ferroelectric properties of polycrystalline sol-gel derived Pb(Nb, Zr, Ti)O3 (PNZT) thin films with SrRuO3 (SRO) electrodes. The processing temperature of the bottom electrode was varied between 550 and 850 °C. The polarization of the ferroelectric capacitors was dependent on the processing temperature of the bottom electrode. The capacitors exhibit low switching fields (40 kV/cm), high resistivity (1011 Ω cm at 3 V) and high remanent polarization values (19 μC/cm2 at 3 V), desirable properties for high-density ferroelectric memories. The activation field for these capacitors was measured to be ∼350 kV/cm and the polarization values exhibited a shallow dependence on the pulse width from 1 s to 1 μs. Fatigue, logic state retention, and dynamic imprint tests indicate robust capacitors from a memory viewpoint. © 1999 American Institute of Physics.

Notizen: We report results of high-speed polarization relaxation measurements in ferroelectric thin film capacitors. Polarization relaxation has been reported to occur in two distinct time regimes, one for relaxation times in the range of a few milliseconds and a second for longer relaxation times. We find that the polarization relaxation in the first regime is governed by at least two different physical processes, namely depoling fields and the activation field for switching. Using prototypical epitaxial PbZr0.2Ti0.8O3 and Pb0.9La0.1Zr0.2Ti0.8O3 test capacitors, we demonstrate the effect of film microstructure and switching speed on the relaxation dynamics in the first regime. © 1998 American Institute of Physics.

Notizen: We have found that, with proper pregrowth surface treatment, 6H-SiC single-crystal films can be grown by chemical vapor deposition (CVD) at 1450 °C on vicinal (0001) 6H-SiC with tilt angles as small as 0.1°. Previously, tilt angles of greater than 1.5° were required to achieve 6H on 6H at this growth temperature. In addition, 3C-SiC could be induced to grow within selected regions on the 6H substrate. The 3C regions contained few (or zero) double-positioning boundaries and a low density of stacking faults. A new growth model is proposed to explain the control of SiC polytype in this epitaxial film growth process.

Notizen: Both 3C-SiC and 6H-SiC single-crystal films can be grown on vicinal (0001) 6H-SiC wafers. We have found that oxidation can be a powerful diagnostic process for (1) "color mapping'' the 3C and 6H regions of these films, (2) decorating stacking faults in the films, (3) enhancing the decoration of double positioning boundaries, and (4) decorating polishing damage. Contrary to previously published oxidation results, proper oxidation conditions can yield interference colors that provide a definitive map of the polytype distribution for both the Si face and C face of SiC films. Defects were more effectively decorated by oxidation on the Si face than on the C face.

Notizen: Current practice  Sedation of children in requiring intensive care in Europe has favoured a combination of a sedative drug, mainly midazolam, and an analgesic, usually morphine. In North America fentanyl is often employed as the first line analgesic. Postoperatively, the need for analgesia is obvious but for other patients analgesic drugs are used as an adjunct to spare the dose of a pure sedative. Propofol, although an undoubtedly useful drug, has been used widely albeit with concerns regarding its safety in children (1) and even in adults (2). This debate was curtailed recently (3) and is no longer a viable alternative, at least in the UK. However, even the more established drugs are not free from generating problems: Tolerance and Tachyphlaxis Classically seen with local analgesics and opioids, this is also observed with midazolam (4) and dose rate does not relate reliably to adequacy of sedation (5) Withdrawal This is seen with almost all classes of drugs used in sedation in PICU. Not only as ‘expected’ with opioids and the benzodiazepines, but also with propofol (6), isoflurane (7) and barbiturates (8). Many of these reactions may be mediated through NMDA receptors and this can be modified by the use of ketamine (8). Some of these reactions can have long drawn out symptoms and necessitate tapering regimens that may fast up to three weeks (8, 9, 10). The incidence of such phenomena varies from 17 to 35% in published data, particularly where midazolam was concerned (9,10) Immune effects Morphine has titratable effects in decreasing the proliferation of stimulated human lymphocytes, and it also suppresses killer cell cytotoxicity. Selective α, κ and δ agonists decrease immunoglobulin production by B lymphocytes (1l).Sedative sparing strategies  Clinical observation systems should be objective and reproducible in a quantitative form so that assessments of sedation can be made independently of varying individuals or units. The COMFORT score is validated but has 5 scores in each of 8 categories (12) so we use a simpler version in our unit (13). The EEG is difficult to interpret and suppression of waveforms is non linear with concentration of propofol (14). In paediatric patients we found that propofol levels varied widely, given the same dose, and that there was no correlation between these levels, the EEG and the level of clinical sedation (15).The use of muscle relaxants may reduce the total sedative requirement, but may lead to overdosage where awareness is feared (8). They affect thoracic compliance (16) and lead to more nosocomial infections (16,17), and increase the length of ventilation and length of stay (17). Drug ‘holidays’ can be achieved with reduced length of stay and length of ventilation in adults (18) but this would be problematic in children. However, cycling of drug regimens can be used to good effect. We vary drug regimens every week in a three week cycle, switching from morphine/midazolam (week 1) to ketamine/promethazine (week 2) to clonidine/chloral (week 3). Analgesia can be delivered in short acting form as a bolus for specific procedures, e.g. physiotherapy and as regional blockade, epidurally or spinally, even for thoracic and cardiac procedures (19, 20).Dealing with sequelae  Withdrawal is the most obvious side effect and can be extremely distressing for the child, the parents and for the clinical staff. A good outcome from the acute illness can be frustratingly marred by unwelcome neurological side effects that range from jitteriness, irritability, refusal to feed/drink, to outright myoclonus, ataxia and vomiting (8). Gradual tapering of sedatives is one approach but substitution by a drug from the same class or by a drug from a different class with similar effects can be beneficial.The partial α2 agonist drug clonidine has proved to be very useful in either combating withdrawal or being used as a primary sedative (21, 22). Dexmedetomidine, a drug first used in veterinary anaesthesia, is being evaluated as a first line sedative in adults. It is a pure α2 agonist and may be more prone to hypotensive and bradycardic episodes (23). Clonidine has some α1 activity that may be more protective in this regard.Other agents can be used to combat withdrawal and drugs used in clinical practice include chlorpromazine, lorazepam, oramorph and methadone.Sleep deprivation plays a part in withdrawal. Diurnal variation is disturbed and this is associated with abnormal steroid and melatonin secretion (24).Future Developments for Evaluation  Lorazepam may have advantages over midazolam, with shorter return to consciousness (25).Further study of α2 agonists is needed.Remifentanil as a primary sedative/analgesic agent looks promising, especially in view of its pharmacokinetic profile. We are evaluating its use as a procedural bolus drug.Eltanolone/pregnanolone and other such drugs are being evaluated as a possible substitute for propofol.Regional analgesia is being looked at in our unit, not only for its beneficial effect intraoperatively (20), but as a means of sparing opioid administration on PICU.Melatonin needs evaluating in re-establishing sleep patterns in children in intensive care together with its specific sleep inducing properties (26).